Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene is likely to increase, due to a developing shortage of propylene.
Thus, a process that uses higher alkenes instead of propylene as feed and coproduces higher ketones, such as cyclohexanone, rather than acetone may be an attractive alternative route to the production of phenols. For example, there is a growing market for cyclohexanone, which is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam and nylon 6.
It is known from U.S. Pat. No. 5,053,571 that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a catalyst comprising ruthenium and nickel supported on zeolite beta and that the resultant cyclohexylbenzene can be processed in two steps to cyclohexanone and phenol. Similarly, U.S. Pat. No. 6,037,513 discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide and the peroxide decomposed to the desired phenol and cyclohexanone.
One problem facing the commercial exploitation of these methods of producing cyclohexylbenzene is that generally around 20 wt % of the products of the hydroalkylation process are di- and tri-cyclohexylbenzenes. For the overall process to be economically feasible, it is therefore necessary to convert these polycyclohexylbenzenes into additional useful monocyclohexylbenzene product.
One possible solution to this problem is to transalkylate the polycyclohexylbenzenes with additional benzene, a solution which is addressed in the '513 patent by effecting the transalkylation in the presence of a catalyst containing the same molecular sieve as used in the hydroalkylation catalyst, namely an MCM-22 family catalyst, but in the absence of the metal components on the hydroalkylation catalyst and in the absence of a hydrogen co-feed.
In addition, U.S. Pat. No. 6,489,529 discloses that dicyclohexylbenzene can be converted to the monoalkylated derivative by contacting the dicyclohexylbenzene with benzene in the presence of a catalyst selected from the group consisting of an acidic solid comprising Group IVB metal oxide modified with an oxyanion of a Group VIB metal oxide, TEA-mordenite, and zeolite beta.
In our co-pending PCT Application No. PCT/EP 2008/006072 we have described a process for producing cyclohexylbenzene where dicyclohexylbenzene by-product is transalkylated with additional benzene in a transalkylation reactor, normally separate from the hydroalkylation reactor, over a suitable transalkylation catalyst, such as a molecular sieve of the MCM-22 family, zeolite beta, MCM-68 (see U.S. Pat. No. 6,049,018), zeolite X, zeolite Y, zeolite USY, and mordenite.
The silica/alumina molar ratio of a given zeolite is often variable. For example, certain faujasite zeolites can have varying silica/alumina ratios such as zeolite X which can be synthesized with silica/alumina molar ratios of from 1.5:1 up to 3:1, while that ratio in zeolite Y is from 3:1 to 6:1. In the ultrastable Y zeolite (zeolite USY), which is made by dealuminating zeolite Y, the silica/alumina ratio can be made to exceed the value of 6:1 typical for zeolite Y. The term dealuminating is generally understood to mean the removal of aluminum from the zeolite framework, even where the overall composition of the material is unaltered or only slightly altered because the aluminum removed from the framework remains in the channels and cavities.
For any catalyst to be viable for the commercial scale transalkylation of polycyclohexylbenzenes, the catalyst must not only exhibit significant activity in the conversion of the polyalkylated species but must also selectively produce the desired monocyclohexylbenzene product rather than unwanted by-products. In particular, it is important to minimize the production of methylcyclopentylbenzene since the latter can be difficult to separate from the desired monocyclohexylbenzene product and produces heavy alcohols and ketones in the subsequent oxidation stage.
According to the present invention, it has now been found that certain forms of zeolite USY exhibit a unique combination of a high activity for the conversion of polycyclohexylbenzenes with a high selectivity for the desired monocyclohexylbenzene product and a low selectivity for the unwanted methylcyclopentylbenzene impurity.